if we are going for recovery in RAC database based on Log sequenc#, how do we do that.if its a 3 node RAC we will be having 3 threads and each thread will not have same SEQ# in this situation how do we recover based on sequence#?

Essentially, all recoveries are SCN based. When you recover using an "until time" or a log sequence number, Oracle works out the associated SCN and does the recovery to that SCN. That way, log sequence numbers have no affect, since all this does is allow you to indirectly specify the SCN.